Gas leaks into the main body of a plasma reactor are simple to detect. A "leakback" test will measure the leakage of gases into the evacuated process chamber. If all other gas sources are disconnected, it is obvious that the leak is due to air. But, a N.sub.2 or air leak into the gas lines, the lines bringing the process gases to the reactor chamber, is a more difficult problem to diagnose and measure.
When such a problem is suspected, the appropriate gases are shut off at the source, the gas lines are evacuated and a "leakback" measurement is made on the "system" all the way back to the source. The problem with this technique is that gas supply lines, in a typical wafer fabrication lab, can be fifty to one hundred feet long. A complete evacuation and degassing of such a line can take many hours. Even then, it is sometimes difficult to determine if a particular leakback value on an evacuated line is due to a real leak, or a virtual leak due to the continued degassing of the line.
There are technical ways to analyze this problem. An RGA (Residual Gas Analyzer) will detect the concentration of O.sub.2 or N.sub.2 in any process gas. However, such apparatus is expensive, and is not routinely available in wafer fabrication labs, and has to be specifically hooked into the reactor.
Spectrophotometric techniques are other methods of testing. It is known that different species of gases, when excited by the plasma of the reactor, emit at specific wavelengths. In principle, the measurement of the intensity of the specific wavelength can be used to determine the amount of that species. In practice, spectral intensity measurements are readily obtained on most plasma reactors, as they are all equipped with "endpoint detectors". These are full spectrometers, or single-wavelength band-pass filters that are connected appropriately to measure the intensity of a specific wavelength.
While spectral intensity measurements are readily available, the determination of the concentration of species [X], from the intensity of the wavelength of X (I[X]), is not a straight forward matter. The problem is, that in addition to the concentration of [X], the value of I[X] at any particular time is a function of: the gain setting of the detector; the spectral transmittance of the window through which the I[X] measurement is made, this changes with the state of the reactor (residue buildup on the windows; and the other gases and pressure and RF conditions in the plasma so that there is no way to have a "standard calibration curve" of I[X] vs [X], and thus determine the magnitude of the leak of gas X into the process gas(es).